Mobile vehicles such as tractors and other farm vehicles usually have hydraulically powered steering systems. The hydraulic steering system is comprised of a hydraulic pump, a hydraulic cylinder mounted to pivot the vehicle wheel, a directional flow control valve (or steering valve) for controlling flow from the pump to the cylinder. In most instances the pump will also power auxiliary hydraulic systems such as cylinders or hydraulic motors which control the implements on the vehicle, therefore a steering priority flow divider valve is provided in the hydraulic system. An example of a flow divider valve is illustrated in U.S. Pat. No. 3,916,932 issued to Thorson.
The priority valve divides the hydraulic flow between the steering system and the auxiliary systems of the vehicle. Safety considerations mandate that the priority valve give priority to the flow demands of the steering system over the flow demand of the auxiliary systems. When the total flow demand of the vehicle hydraulic system is beyond the capacity of the pump, the flow demand of the steering system is met first before any flow is delivered to the auxiliary systems.
Often the pump utilized in such a hydraulic system as described above, is a variable displacement pump. In some hydraulic systems the variable displacement pump will have a compensator, wherein the variable displacement pump will only supply the combined flow demanded by the steering system, auxiliary system and leakage. The compensator senses the load pressure of the complete hydraulic system and maintains the variable displacement pump output pressure at a preset level above the load pressure. When there is no load the variable displacement pump idles at the preset standby pressure. However, due to internal flow characteristics of the compensator, the standby pressure usually jumps to a level beyond the preset level when the pump displacement (or the swash angle) is very close to zero.
There are two main problems associated with the above noted system. If the steering wheel is rapidly turned to the left, then to the right, then to left again, such as in an avoidance maneuver, the variable displacement pump will first stroke towards maximum displacement, then rapidly destroke to almost zero displacement, and then again stroke towards maximum displacement.
In the above maneuver the compensator will rapidly go from a lower preset pressure differential to a higher pressure differential pressure as the variable displacement pump's displacement approaches zero. The aforementions fluctuation in compensator differential pressure causes an instantaneous pressure imbalance at the priority valve which is felt as a pulsation in the pump, steering system and the hydraulic lines feeding the steering system. The above noted pressure pulsation is highly undesirable.
When the auxiliary system is also demanding pump flow, the pulsation is significantly diminished because the variable displacement pump is not allowed to go to zero displacement even during an avoidance maneuver.
The priority valve, which is constantly trying to supply the steering system with whatever flow is demanded, can often experience unstable operation. The above instability is manifested in the form of a pressure pulsation which is felt in the vehicle tires during steering operation. The pressure pulsation in the tires is undesirable.